List your item as carefully as possible with lots of photos and possibly even a link to the instrument being played, and make it clear that you're selling as-is. That means you won't accept returns (and unless you ship a piano in place of the guitar you were offering, eBay doesn't insist you accept returns, though they'd like to.) Tell buyers to ask all of their questions before hand. Sell as "Buy it Now" so some noob doesn't jump in at the end of an auction and bid on something they didn't intend to buy then make a stink or cause a problem.

I don't buy some categories of things on eBay, because you can't be sure about the sellers. Ratings are important there, but so few people post ratings any more it's difficult to be sure what to think of some sellers.